This invention relates to the field of sintered electric contact materials particularly suitable for making contacts which must make and break while carrying relatively high power currents.
German Pat. No. 1,029,571 suggests a method for manufacturing sintered electric contact material by atomizing into an oxidizing atmosphere a molten alloy of silver and a non-noble metal, the non-noble metal being one that oxidizes at a temperature less than would result in the silver being oxidized. The result is a powder of spheroidal particles with the non-noble metal internally oxidized within the particles. Powder metallurgical techniques can be used to form the electric contact material from this powder. Metals, such as zinc, cadmium, mercury, tin and/or lead may be used as the non-noble metal alloyed with the silver to comprise the molten alloy that is atomized to form the spheroidal particles.
German Offenlegungsschrift No. 2,011,002 sugggests a silver-metal oxide contact material having a coarse grained silver matrix with the metal oxide in the form of particles having a size less than 0.1 microns, concentrated at the grain boundaries of the silver.
This second suggestion has the disadvantage that the concentration of the metal oxide particles at the silver grain boundaries causes the resulting material to be excessively brittle.
The first suggestion has the disadvantage that the internally oxidized particles obtained by atomization, are spheroidal in shape preventing them from being compressed into moldings of adequately low porosity, the sintered moldings being of high porosity whereas electric contact material should be substantially free from porosity. The oxidized internal non-noble metal component provides such spheroidal particles with a dispersion-hardening effect preventing their deformation by compression to effect closing of the voids resulting from their spheroidal shape.
Furthermore, known techniques for atomizing a silver-non-noble metal alloy to powder form having a particle size below 0.2 mm, results in a yield of such maximum size of only about 50 percent, the balance of the particles necessarily being treated as a reflux which must be returned for remelting to be again atomized. This particle size cannot be exceeded to any substantial degree if the powder is to have good molding properties.
The object of the present invention is to provide a method for making electric contact material consisting of sintered internally oxidized silver-metal oxide powder particles, which avoids the foregoing undesirable features.